Splendid isolation: South America was an island for millions of years, fostering an evolutionary explosion of unique mammal species

Natural History, June, 2009 by John J. Flynn

MENTION Australia, and kangaroos, koalas, and platypuses spring instantly to mind. Madagascar? Lemurs, of course! What about South America's native mammals? Llamas, alpacas, and jaguars, right? Think again. Like many familiar South American animals, those species represent descendants of relatively recent invaders from North America. South America's original native mammals were far more unusual by today's standards: elephant-size ground sloths, tanklike armadillo relatives weighing as much as two tons, tiny burrowing marsupials, and hundreds of hoofed species that looked like impersonations of modern rhinos, horses, and camels.

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South America, like Australia and Madagascar today, was largely isolated from other landmasses by ocean barriers for millions of years. The importance of such geographic isolation in generating new species is a basic tenet of modern evolutionary theory. Species can diverge rapidly from their mainland relatives once geographic separation has occurred--a process known as "allopatry." This "island effect" has repeatedly led to a remarkable profusion of unique animals at a variety of spatial scales, ranging from small islands with just a few specialized species to island continents like Australia and ancient South America, dominated by hundreds of forms found nowhere else.

On a relatively modest scale, the isolation of small islands off the mainland may lead to dwarfism or gigantism in species, such as the evolution of(now extinct) dwarf mammoths on the California Channel Islands, or giant rabbits and shrews on some Mediterranean islands [see "The Island Sweepstakes," September 1986]. In fact, my colleagues and I recently described a new species of dwarf water buffalo, no more than a few tens of thousands of years old, from the Philippine island of Mindanao. It stood only two and a half feet high at the shoulders and weighed about 350 pounds--an amazing miniaturization considering that its ancestors were six feet tall and weighed a ton.

At the intermediate scale are large islands such as Madagascar, which began to separate from mainland Africa at least 160 million years ago and arrived close to its present position some 120 million years ago. Although early mammals came along for the ride, they later were replaced by a few immigrant groups of modern mammals that made it to Madagascar from the mainland across a 250- to 600-mile-wide channel. Four land-dwelling mammal lineages that still populate Madagascar, and at least another two groups now extinct there, participated in those rare events, termed "sweepstakes dispersals"--as did bats, which can more easily disperse over water barriers. Of the four terrestrial founders, one was ancestral to the island's living species of endemic carnivorans, seven in total, which resemble cats, civets, and mongooses. Another is represented by twenty-four living rodent species; thirty species of hedgehog-like tenrecs trace their origin to a third; and a fourth founder diversified into the fifty or more living (and fifteen to twenty extinct) species of lemurs--primates that do not occur anywhere else today.

But on a grander scale, the most striking example of mammal evolution in an isolated setting may be South America, in part because of its wide latitude spread and great variety of habitats, including equatorial rain forests, the high Andes Mountains, windswept grassy pampas, and the plains of Patagonia. Although today connected to North America by the narrow Isthmus of Panama, South America was an island continent for the bulk of the Cenozoic era, the so-called Age of Mammals (from 65 million years ago to present). That "splendid isolation," a phrase adopted almost three decades ago by the famed American Museum of Natural History paleontologist and evolutionary biologist George Gaylord Simpson, led to the evolution of a wondrous array of plants and animals--perhaps more species than on any other landmass. Among them were ancient lineages of mammals, the vast majority of which went extinct without leaving any descendants. The opossums, armadillos, anteaters, and tree sloths living there today only provide a hint of the continent's homegrown treasures.

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To begin at the beginning, the earliest evidence of mammals, dating at least as far back as the early Jurassic period, about 195 million years ago, comprises fossils of creatures resembling small shrews that coexisted with the dinosaurs. At the time there was only a single supercontinent, Pangea, that had begun to fragment, and the climate was much warmer. Mammals diversified as Pangea continued to break apart, first into a northern supercontinent, Laurasia, and a southern one, Gondwana. These long retained a few intermittent connections, but between 180 and 34 million years ago, various parts of the southern supercontinent gradually broke apart. During the same period, Laurasia lay across the Northern Hemisphere, but later split into North America and Eurasia.